MEMS sensors are micro-electromechanical systems, which are applied for metrological registering of a measured variable, e.g. a pressure, a mass- or a volume flow, a density, a viscosity, a temperature, a pH-value or an electrical conductivity.
MEMS sensors are regularly constructed of layers, especially silicon layers, arranged on one another, and by applying methods usual in semiconductor technology, such as e.g. etching processes, oxidation methods, implantation methods, bonding methods and/or coating methods. In such case, the individual layers, as well as, in given cases, connecting layers, e.g. insulation layers, provided between adjoining layers, are prepared, and structured, corresponding to the functions assigned to them in the sensor.
MEMS sensors regularly comprise components, which can be exposed to a mechanical load. An example, in such case, are functional elements of electromechanical transducers integrated in the MEMS sensor. The functional elements are exposed to a mechanical load dependent on the measured variable to be registered. The mechanical load is converted by the transducer into an electrical variable dependent on the measured variable.
Mechanical loads bring about unavoidable stresses, which mechanically affect individual sensor components and/or components connected with individual sensor components. This is not a problem as long as the loads do not exceed a load limit, frequently referred to as the overload resistance, predetermined for the sensor. As a practical matter, however, sensors are continually being exposed to overloads.
The danger of overloading is especially great in the case of pressure difference sensors, which are applied for measuring pressure differences Δp between two pressures p1, p2 large in comparison to the pressure difference Δp. Since these sensors must be sensitive enough metrologically to register the pressure difference Δp superimposed on the system pressure corresponding to the smaller of the two pressures, a problem arises, above all, in the case of unilateral overloads, when only one side of a measuring membrane of the pressure difference sensor is supplied with a high pressure, and there is no counterpressure acting on the oppositely lying side of the measuring membrane.
In such case, sensors, which have regions with well-developed edges, are especially sensitive to mechanical loading, since abrupt edges lead to stress concentrations, which in the case overloading can bring about stress cracking or even stress fractures.
An example of this involves piezoresistive pressure sensors, which have a measuring membrane loadable with a pressure to be measured. These pressure sensors are regularly produced, in which case a cavity is etched into a silicon wafer by means of an anisotropic etching method, e.g. an anisotropic etching method performed with potassium hydroxide (KOH). In such case, an edge of the sensor chip surrounding the cavity forms a carrier, which surrounds the measuring membrane exposed by the cavity. However, anisotropic etching methods produce abrupt edges at the transition between the measuring membrane and the carrier, where stress concentrations occur.
For solving this problem, German Patent, DE 10 2008 035 017 A1 describes producing the measuring membrane exposing cavity by means of a combination of anisotropic and isotropic etching methods. In such case, there is produced with the anisotropic etching method a cavity, which tapers conically for increasing the mechanical stability in the direction of the measuring membrane. Then, in a following, isotropic etching procedure, the abrupt edges arising in the anisotropic etching are rounded at the transition with the measuring membrane. The rounded edges reduce stress concentrations.
An alternative form of embodiment of a piezoresistive pressure sensor is described in German Patent, DE 10 2007 010 913 A1. This has two layers of silicon, which are connected with one another via a connecting layer, namely an oxide layer, arranged between the two layers. In the case of this pressure sensor, piezoresistive elements are provided on one of the layers and the second layer has a cavity, via which a region of the first layer forming the measuring membrane and the oxide layer connected therewith are exposed. In order to be able metrologically to register also small pressures with a small linearity error, a groove is provided in the oxide layer on the side of the measuring membrane lying opposite the piezoresistive elements and serves to concentrate the stresses produced by the pressure acting on the measuring membrane at the locations, where the piezoresistive elements are located. For increasing the strength of the sensor, the groove has a rounded cross-sectional geometry, and the cavity in the second layer has preferably a lateral surface conically tapering in the direction of the oxide layer.
The combination of a stability increasing, conically tapering cavity with a rounded transition effected e.g. by a rounded groove or a rounded edge assumes, however, that sufficient space is available for a conically tapering and therewith unavoidably bigger cavity on its open side, and that the rounded edge is accessible in measure sufficient for using an isotropic etching method.
Both assumptions are fulfillable, when the cavity is located in an outer layer, thus an outer layer accessible from outside of the MEMS sensor.
The stress concentrations limiting overload resistance of MEMS sensors can, however, also occur at locations in the interior of the sensors, locations which are not directly externally accessible. This situation is present in the case of MEMS sensors, which have a cavity enclosed in the interior of the sensor, where at least one portion borders a layer, which, in given cases, can be exposed to mechanical loadings. An example of this is in pressure sensors with a pressure chamber enclosed under a measuring membrane contactable with a pressure. A further example is formed by furrows, e.g. an isolation moat, extending through an inner layer of a sensor and surrounding a portion of the inner layer connected with one of the two outer layers and spaced from the other. There are also MEMS sensors, e.g. capacitive pressure sensors, in the case of which the aforementioned examples occur in combination with one another.
German Patent, DE 103 93 943 B3 describes a pressure difference sensor,
which has a plurality of layers, especially silicon layers, arranged on one another,
wherein the layers include at least one inner layer, which is arranged between a first layer and a second layer, and
there is provided in its inner layer at least one cavity extending perpendicularly to the plane of the inner layer, through the inner layer, on which cavity there borders externally at least sectionally a region of the inner layer forming a connecting element and connecting the first layer and the second layer.
The pressure difference sensor includes a first layer surrounding a measuring membrane and arranged between two platforms. Each platform is connected with the first layer to enclose a pressure chamber, and includes an inner layer and a second layer connected therewith via a connecting layer. The inner layers are divided by a cavity embodied as an isolation moat into an outer region forming the connecting element and an inner region externally surrounded on all sides by the connecting element. The inner regions serve as electrodes and are, in each case, spaced from the measuring membrane by a cavity in the inner layer connected with the isolation moat. Each electrode forms together with the first layer serving as counterelectrode a capacitor with a capacitance dependent on the pressure acting on the measuring membrane.
In order to configure MEMS sensors as small and as stably as possible, it is important to keep cavities in the interior of MEMS sensors small. Toward this end, cavities in inner layers of MEMS sensors are preferably externally limited by lateral surfaces, which extend essentially perpendicularly to the first and second layers. This leads to essentially right angled transitions from the connecting element to the first layer and from the connecting element to the second layer. Correspondingly, forces acting on the first layer, the second layer and/or the connecting element in the transitional regions lead to stress concentrations, which limit the overload resistance of the sensor.